Messages in this thread | | | Subject | Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug | From | Peter Zijlstra <> | Date | Fri, 26 Nov 2010 13:10:08 +0100 |
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On Fri, 2010-11-26 at 13:00 +0100, Heiko Carstens wrote: > plain text document attachment (001_printk_preempt.diff) > From: Heiko Carstens <heiko.carstens@de.ibm.com> > > wake_up_klogd() may get called from preemtible context but uses > __raw_get_cpu_var() to write to a per cpu variable. If it gets preempted between > getting the address and writing to it, the cpu in question could be offline if > the process gets scheduled back and hence writes to the per cpu data of an offline > cpu. > > No idea why that behaviour was introduced with fa33507a "printk: robustify > printk, fix #2" which was supposed to fix a "using smp_processor_id() in > preemptible" warning. > > Let's use get_cpu_var() instead which disables preemption and makes sure that > the outlined scenario cannot happen. > > Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com> > --- > kernel/printk.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > --- a/kernel/printk.c > +++ b/kernel/printk.c > @@ -1087,8 +1087,10 @@ int printk_needs_cpu(int cpu) > > void wake_up_klogd(void) > { > - if (waitqueue_active(&log_wait)) > - __raw_get_cpu_var(printk_pending) = 1; > + if (waitqueue_active(&log_wait)) { > + get_cpu_var(printk_pending) = 1; > + put_cpu_var(printk_pending); > + } > } > > /** >
But but but, the cpu can still be offlined between writing this state and the next tick happening, right?
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